Emulsions comprised of a dense gas, a liquid polar solvent and a surfactant have been recognized to be highly useful processing fluids for a variety of purposes such as, for example, use as a solvent for extracting soluble substances from fluids. The solute which is extracted may typically be a flavor, fragrance, pharmaceutical, chelated metal or nonvolatile hydrophilic substances such as proteins.
Using a dense gas/liquid/surfactant emulsion as a processing fluid, such as a solvent for example, has a number of advantages. One is that it is easier to obtain solvent free products because of the high vapor pressure of a dense gas. The dense gas is easily vaporized without requiring a great deal of heating. The resulting moderate processing temperature preserves the integrity of heal sensitive products. Energy is also conserved.
A dense gas, for example CO.sub.2 at a density greater than about 0.3 g/cc, has additional advantages. Carbon dioxide is non-toxic, non-inflammable, inert to most materials and is inexpensive. Dense gases have low viscosities which lead to high diffusion and mass transfer rates.
A disadvantage of dense gases as solvents is that their dissolving power is generally lower than that of conventional liquids. For example, dense CO.sub.2 dissolves primarily low-polarity materials, but not high-polarity materials such as proteins, carbohydrates and ionic compounds. However, these high-polarity compounds dissolve in a polar solvent such as water.
An emulsion of a liquid polar solvent, such as water, in a dense gas, such as compressed CO.sub.2, provides a processing fluid with some of the properties of both fluids. For example, the dispersed micelles of water provide dissolving power for proteins, while the dense CO.sub.2 provides favorable bulk properties such as low viscosity.
Dense gas/liquid emulsions eventually separate back into layers of the two phases. However, the emulsions become stable dispersions for practical purposes when an appropriate surfactant is added to the mixture.
The dense gas/liquid emulsions become more useful as processing fluids when they can be demulsified to enable recovery of the surfactant and dense gas. A product, such as a protein dissolved in liquid water, can then be recovered without being contaminated with the surfactant. The dense gas and expensive surfactant can then be recycled to make the process economically feasible. Prior methods and apparatus for accomplishing demulsification of dense gas/liquid/surfactant emulsions are not ideally suited for this purpose.
For example, International Patent Application No. PCT/US96/18168, published May 22, 1997 (International Publication No. WO 97/1823) describes an extraction process using dense CO.sub.2 /water/surfactant as the processing fluid. The carbon dioxide and surfactant are to be recycled. The described method for recovering the CO.sub.2 includes complete vaporization of the CO.sub.2 by decompression to its gaseous state. This leaves a residue of liquid water mixed with surfactant. The surfactant is recovered from the residue by decanting or other physical means known to the art. Some lost surfactant remains in the product after decanting. Recycling requires a makeup step to compensate for the lost surfactant. The decompression and subsequent recompression of the CO.sub.2 for recycling also complicates the process and increases costs.
The present invention is directed to overcoming one or more of the problems discussed above.